Hyperhomocysteinemia (HHcy) is an important non-lipid risk factor for cardiovascular disease (CVD). At present, our mechanistic knowledge of HHcy's correlation with dyslipidemia and the development of atherosclerosis is limited. Notwithstanding, it has been suggested that HHcy affects hepatic lipid metabolism, thereby contributing to fatty liver, a condition described in humans and animals with HHcy. Our previous findings suggested that plasma HHcy levels are significant negative correlated with plasma HDL-C and apolipoprotein AI (apoA-I), a predominant structural protein in HDL particles and cofactor for HDL maturation, in patients with coronary heart disease (CHD) and in atherosclerosis mice. We also found that severe HHcy is associated with increased HDL-C turnover and increased protein levels of hepatic/vascular endothelial lipase (EL), an HDL degradation enzyme in mice. Because the mechanistic link between HHcy and HDL dysfunction has never been studied, we plan to investigate the role and mechanism of HHcy-induced HDL dysfunction in our newly established severe HHcy models with double gene deficiency for Cystathionine ?-synthase (CBS) which degrade homocysteine (Hcy), and either apolipoprotein E (ApoE) or low density lipoprotein receptor (LDLR), and with an inducible human CBS gene (Tg-hCBS ApoE-/- Cbs-/-, Tg- hCBS Cbs-/-, and Ldlr-/- Cbs-/+). These set of transgenic mice can circumvent the potential limitations of different model system and are valid for assessing the role and mechanistic links of HHcy-HDL metabolism with/without hyperlipidemia and with/without apoE deficiency. The central hypothesis to be tested in this proposal is that HHcy causes ApoA1 reduction and EL activation, resulting in reduced HDL maturation and increased HDL degradation, leading to impaired reverse cholesterol transport (RCT) contributing to the development of atherosclerosis. We proposed in Aim 1 to determine the effect of HHcy on HDL biosynthesis, catabolism, and function in our newly three developed HHcy models, in Aim 2 to identify molecular targets and underlying biochemical basis mediating HHcy-altered HDL metabolism, and in Aim 3 to reverse HHcy, ApoA1 deficiency or EL activation, and to examine the effect on HDL-raising/function and atherosclerosis. The goal of this proposal is to investigate the role of HHcy in HDL metabolism, in the hope to identify the underlying molecular mechanisms and novel therapies. If the key steps in Hcy-induced dyslipidemia and atherosclerosis can be identified, new genetic or pharmacological therapeutic targets can be utilized for treatment.